The invention relates to a controllable direct-current motor, in particular an EC motor, which has a modified rotation-speed/torque characteristic in comparison to a conventional motor, for example as in the case of a motor which is coupled to a mechanical two-gear variable-speed gearbox.
In the case of electric tools in the prior art, motors are in general coupled to variable-speed gearboxes, in order to cover a wider rotation-speed/torque range. Two-gear machines, three-gear machines and in some cases even four-gear machines are provided for this purpose.
However, the use of a variable-speed gearbox involves considerable complexity, associated with an additional weight and space requirement. Furthermore, mechanical gearboxes must not be switched when under load during operation. There is therefore no capability to match the optimum gearbox step-up ratio to the current working load during operation.
DE 10 2004 027 635 A1 discloses an electric tool having a drive unit with a motor which comprises a rotor with a permanent magnet and a stator, with a motor controller being provided which is designed such that it controls the motor in a first rotation-speed range on the basis of a voltage-controlled operating mode, and controls it in a second rotation-speed range, which is adjacent to a first rotation-speed range in the direction of a higher rotation speed, on the basis of a field-weakening mode. In the first rotation-speed range, the motor is operated on a voltage-controlled basis, with the motor controller in this operating mode applying a voltage to the motor in order to predetermine the rotation speed of the motor. When a specific rotation speed is reached, the motor switches to a different operating mode, which is referred to as the field-weakening mode. In this case, the induced back-emf is weakened, thus resulting in a higher rotation speed with a lower torque. Overall, therefore, the motor can be designed for a lower mean rotation speed, while switching takes place to the field-weakening mode in order to achieve higher rotation speeds. This allows an extended rotation-speed/torque range to be achieved without using a variable-speed mechanical gearbox.
However, this represents a special case only for very specific applications, in which a user must switch to the field-weakening mode in order to increase the rotation speed.